I read a response that you had posted on the internet with regards to the Epics XL Coulter flow cytometer. I have an equally naive question. When the instrument is set to acquire on log gain for FL 1 channel and the statistics printed include "Mn I X" what is that value, is it a linear "regular number" and can I subtract the Mn I X of the control from the sample to get a net Mn I X? Dolly- In Coulter's unique terminology Mn I X is the Geometric mean channel of fluorescence intensity of data plotted on the X axis of a histogram (as opposed to Mn X, which is the arithmetic mean). For most applications using monoclonal antibodies (and whenever you use log fluorescence) you want to use Mn I X for your average. The geometric mean can be calculated by taking the log of the fluorescence values, averaging them and then taking the antilog of the average. Subtracting one geometric mean from another to get net fluorescence sounds a bit shaky and I'm not sure why you want to do this. Chances are that your "background" has a lot of events piled up in the first channel. If this is the case the mean value (geometric or arithmetic) isn't a very good summary statistic. Perhaps you could compare median values of your background and experimental conditions. Median values are not effected by things like pile-ups in the first channel or long tails. Hope this helps. Albert
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